In distributed power generation systems, such as solar installations, DC power is generated and converted into AC power for delivery to the power grid. Conversion from DC to AC power is performed by a power inverter. A typical power inverter is computer controlled, using a microprocessor or other suitable system controller. The system controller transitions the inverter between several states such as shut-down, sleep, online and fault-handling states based on the AC and DC voltage and current conditions. AC and DC contactors are used to disconnect the inverter from the AC and DC sources. The AC and DC contactors are controlled by the system controller through Solid State Relays (SSRs).
A fault situation can occur when the AC contactor is unknowingly open but the system controller still commands the power matrix of the power inverter to switch. This situation will cause high and distorted magnetizing current to flow into an isolation transformer of the power inverter, eventually damaging the isolation transformer and the power inverter.